1. Field of the Invention
The present invention relates to a laminated thin film capacitor and a method for producing the same.
2. Description of the Related Art
Barium titanate (BiTaO.sub.3) having a perovskite crystal structure is widely used as a capacitor material since it is excellent in dielectric properties and has a high resistivity. Since barium titanate is characterized in that its dielectric constant in the a axis is larger than that in the c axis which is a polarization axis, it can provide a capacitor having good dielectric properties when its a axis is orientated preferentially in a direction perpendicular to a plane of an electrode, that is, in the (100) plane.
Strontium titanate (SrTiO.sub.3) having a perovskite crystal structure is a cubic system crystal and paradielectric at a temperature of about 110K or higher. While a ceramic comprising SrTiO.sub.3 has a lower dielectric constant than a ceramic comprising BaTiO.sub.3 having the same crystal structure, the former has better temperature characteristics and smaller dielectric loss than the latter. When a Curie temperature is shifted by the addition of a shifter such as barium to SrTiO.sub.3, a ceramic which is paradielectric and has a high dielectric constant at room temperature is obtained. This kind of ceramic is extensively used in a capacitor for high frequency and high voltage.
A composite perovskite structure compound having a composition of Pb[(Mg.sub.1/3 Nb.sub.2/3).sub.1-x Ti.sub.x ] which is a composite material of Pb(Mg.sub.1/3 Nb.sub.2/3) being one of typical relaxation type ferrodielectric materials and lead titanate (PbTiO.sub.3) having the same perovskite crystal structure as BaTiO.sub.3 or SrTiO.sub.3 has a larger specific dielectric constant and better direct current bias characteristics than a BaTiO.sub.3 type ferrodielectric material. Therefore, this composite perovskite structure compound is used in a small laminated capacitor having a large capacity.
To accommodate a capacitor to miniaturization and high density loading of an electronic device, down-sizing and increase of capacity of a chip capacitor are being progressed. Increase of the capacity can be achieved by the use of a dielectric material having a large dielectric constant or increase of the number of laminated layers by making each dielectric layer thin. For example, a method for producing a laminated ceramic capacitor is described in Electronic Ceramics, No. 103 (1990) pages 57-61. The disclosed method for laminating dielectric layers and metal electrode layers is as follow:
Powder of a dielectric material such as BaTiO.sub.3 is compounded, mixed and dried. A slurry of the dielectric material powder is prepared and formed in a thin film sheet. An internal electrode paste of an electrode material such as palladium is printed on the sheet. These steps are repeated for determined times to alternately laminate the internal electrodes and the sheets. Thereafter, the laminate is cut to a desired size, and heated, whereby the ceramic and the metal are simultaneously sintered.
However, in the above lamination method, there is a limitation on a thickness of the dielectric layer. That is, since a particle size of the dielectric material powder which is a raw material of the dielectric layer is about 1 .mu.m, some problems arise in evenness of the film thickness or insulation between the electrodes when the dielectric layer having a thickness of 3 .mu.m or less is formed by preparing the slurry of the powder, making the sheet from the slurry and sintering the sheet. To produce a chip capacitor having the large capacity, the number of laminated layers should be increased. However, when the number of laminated layers is increased, a size of a chip is enlarged and the number of production steps increases, so that the reliability of the capacitor may be deteriorated. In addition, since the sintering step is necessary after the lamination in the above method and the sintering temperature is as high as about 1200.degree. C., an electrode material is limited to a high melting point metal such as platinum.
Accordingly, a method for producing a laminated capacitor having good characteristics at a low temperature has been sought.